The invention relates generally to motor drive systems, and more specifically, to systems and methods to pre-charge the motor drive systems.
In the field of power electronic devices, a wide range of circuitry is known and currently available for converting, producing and applying power to loads. Depending on the application, such circuitry may convert incoming power from one form to another as needed by the load. In a typical motor control application, for example, a rectifier converts alternating current (AC) power (such as from a utility grid or generator) to direct current (DC) power. Inverter circuitry can then convert the DC signal into an AC signal of a particular frequency desired for driving a motor at a particular speed. The inverter circuitry typically includes solid state power electronic switches, such as insulated-gate bipolar transistors (IGBTs), controlled by drive circuitry. Often, power conditioning circuits, such as capacitors and/or inductors, are employed to remove unwanted voltage ripple on the internal DC bus.
Often, at the first application of AC power to the motor drive circuit detailed above, the circuit will draw high levels of current due to the charging of the power conditioning capacitors. To avoid a high in-rush current at start-up, a typical motor drive may also include a pre-charge circuit, which applies a smaller initial current to the DC bus just prior to start-up to charge the capacitors. The number and rating of the capacitors may be selected based upon expected loading (e.g., via motion profiles, system load analysis, etc.), to provide for and to reduce variability in the bus voltage during operation. Once selected, the capacitance of the power conditioning capacitors is relatively constant, even though the power demand of the motor drive may not be constant over time. In addition, changing the capacitance of the power conditioning capacitors may involve manually removing and replacing portions of the pre-charge circuit, which may be time-consuming and generally inefficient.